Among essential elements for growth of a plant, carbon, hydrogen, and oxygen are supplied from the air and water. Moreover, three elements of nitrogen, phosphorus, and potassium are absorbed from soil. However, since the existence thereof in soil is relatively small as compared with the amount to be absorbed by a plant, they tend to be depleted and effects easily appear when they are externally supplied, so that they are called as three elements of fertilizer. For plants, other than the three elements of fertilizer, there are further essential elements, which specifically include magnesium, sulfur, and calcium classified into major elements and manganese, boron, iron, zinc, copper, molybdenum, chlorine, and nickel classified into minor elements. Moreover, also in human being, deficiency diseases have been known for iron, zinc, copper, manganese, and vanadium. In particular, iron is depleted in an extremely wide distribution as it is said that one third of women suffer from potential iron-deficiency anemia.
Iron (Fe) is involved in the activation of an enzyme as a divalent iron ion (Fe2+). Moreover, iron enters into a porphyrin ring to be transformed into heme and heme is present at active centers of various enzymes. Heme iron-containing proteins include cytochromes, peroxidases, and catalases, and iron-sulfur proteins include ferredoxins, which are involved in mainly oxidation-reduction reactions and electron transfer reactions. Chlorophyll is a substance wherein magnesium ion enters in a porphyrin ring. With regard to a protoporphyrinogen synthetic enzyme and a protochlorophyllide synthetic enzyme as precursor substances of the porphyrin ring synthesis, iron is considered to be involved in the control at a gene level. Therefore, when iron is deficient, the synthesis of porphyrin is inhibited and chlorophyll is not synthesized, so that a plant exhibits chlorosis, cannot perform photosynthesis, and finally is blighted.
Copper forms a constitutional component of tissues or a part of enzymes involved in metabolisms in a plant body. Common symptoms of its deficiency include chlorosis, necrosis, curly leaves, and the like. Plants exhibiting copper deficiency in farm fields include cultivated plants such as barley, wheat, alfalfa, lettuce, carrot, onion, tomato, tobacco, and citrus.
Manganese has physiological actions such as photosynthesis, respiration, and activation of oxygen and, in many plants, respiration increases as the amount of manganese increases in the plant bodies. When manganese is deficient, chlorosis is observed on leaves.
With regard to zinc, a large number of zinc enzymes and the like have been found and many in vivo reactions involving zinc are also known. As symptoms of its deficiency, suppression of elongation growth, inhibition of protein synthesis, and the like are observed at various sites of a variety of plants.
On the other hand, also in human being, importance of mineral intake has been perceived. According to Nihonjin no Shokuji Sesshu Kijun (Dietary Intake Standard of the Japanese) (Non-Patent Document 1), for example, a new index “desired amount” is set for calcium as a nutrient to be increased and also estimated average necessary amounts and recommended amounts are determined for other minerals. Moreover, osteoporosis has been hitherto pointed out owing to deficient intake of calcium but it has been found that only ingestion of calcium is not a sufficient countermeasure and magnesium and the like are also necessary.
However, at present, the intake of magnesium and calcium is about 100 mg lower than the recommended amount in Dietary Intake Standard of the Japanese. Moreover, in the other minerals, though copper is sufficient, the intake of iron and zinc is lower than the recommended amounts in sex and/or some ages.
As methods for ingesting minerals, there may be mentioned a method for ingestion with supplements and a method for ingestion with meals. In general, minerals have characteristics that the width of zone of appropriate intake is narrow and a balance between minerals is apt to be disrupted. The method for ingestion with supplements causes a problem that overdose disorder owing to excessive ingestion of a specific mineral occurs. Mineral overdose induced by the method for ingestion with meals is rare other than the case of sodium overdose with sodium chloride and it is easy to ingest minerals with good balance. Accordingly, as a method for ingesting minerals, it is preferred to ingest minerals with meals in just proportion.
Since vegetables abundantly contain various minerals and vitamins, vegetables are main sources of minerals in meals. However, according to Shokuryo Jukyu Hyo (Food Supply and Demand Table) (Non-Patent Document 2), it is reported that vegetable intake per day has decreased with the change in dietary habit and only an average of 250 g of vegetables is ingested although 350 g thereof per day is required. Thus, in order to increase mineral intake, it is desired to increase the mineral content in vegetables.
As a technology for enhancing metal component absorption in a plant, it has been reported that benzoic acid and/or benzoic acid derivatives enhance absorption of potassium ion (Patent Document 1). Moreover, there is a report that absorption of calcium ion is enhanced by spraying on the surface of leaves together with an aqueous amino acid solution (Patent Document 2) but any substance is not known, by which enhanced absorption of the other metal component(s) is observed.    Patent Document 1: JP-A-2002-284607    Patent Document 2: JP-A-2001-192310    Non-Patent Document 1: Nihonjin no Eiyo Shoyouryo-Shokuji Sesshu Kijun-Sakutei Kentoukai, Nihonjin no Shokuji Sesshu Kijun (Dietary Intake Standard of the Japanese) (2005)    Non-Patent Document 2: 2004, Ministry of Agriculture, Forestry and Fisheries, General Food Policy Bureau, March 2006: Shokuryo Jukyu Hyo (Food Supply and Demand Table)